As for increasing conductivity to use less concrete: the thinner the layer of
concrete, the less the thermal conductivity matters. The PSD Handbook gives
the following values for energy stored in concrete on a daily basis in a
passive solar wall directly irradiated by the sun:
1" - 2.5 BTU/ft2-F
2" - 4.99
3" - 7.37
4" - 9.47
6" - 11.94
8" - 12.14
You can see that it is darn linear to 4 inches! So increasing conductivity
wouldn't make sense until you got to an eight inch wall.
Marc Rosenbaum
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